The purpose of the research is to investigate a new concept on the regulation of myocardial energy metabolism during the stress of coronary underperfusion. It is hypothesized that the enzyme that hydrolyzes AMP to adenosine (5'-nucleotidase) is critical for tissue survival during ischemia because it mediates the interconversion of the myocardial adenylate system (ATP, ADP, AMP) between an open state and a closed state. A closed system (ATP, ADP and AMP have low membrane permeability) would be fatal during underperfusion, because the products of ATP breakdown (ADP and AMP) would accumulate in the cell causing a precipitous fall in the free energy of ATP hydrolysis (Gibb's free energy or DG). Therefore, we will investigate whether 5'-nucleotidase is upregulated during the onset of underperfusion, converting the adenylate system to an open state that allows the products of ATP breakdown to leave the system (adenosine has high membrane permeability). During more sustained underperfusion, w e will test whether 5'-nucleotidase is downregulated, returning the adenylate system to a closed state, to preserve ATP. It is hypothesized that the interconversion between an open and closed adenylate system, in response to changing conditions during underperfusion, is critical for long term tissue survival when blood flow is inadequate, termed "myocardial hibernation". We will also investigate whether AMP is exported from the cell during hypoxia to preserve high energy phosphate potential by a transport system distinct from that used to move nucleosides across the membrane. Finally, we will investigate whether the endothelial nucleoside transporter has a differential affinity for adenosine and inosine. The lower membrane permeability of inosine relative to adenosine may enable the cell to conserve adenosine as inosine levels increase in the cell with hypoxia or underperfusion. If inosine binds to the transporter with an affinity equal to or greater than that of adenosine, the eleva ted intracellular level of inosine will preferenially occupy the transporter and thus prevent adenosine from being lost. The research involves a unique combination of rabbit heart data on high energy phosphates (NMR spectroscopy) and coronary venous purines (biochemical techniques), and an integrated analysis using a mathematical model describing the pathways of myocardial phosphoenergetics and adenosine.